Education: Activities & Documents

Electrolysis of Salt Water

In chemistry, electrolysis is a method of separating bonded elements and compounds by passing an electric current through them. An ionic compound, in this case salt, is dissolved with an appropriate solvent, such as water, so that its ions are available in the liquid. An electrical current is applied between a pair of inert electrodes immersed in the liquid. The negatively charged electrode is called the cathode, and the positively charged one the anode.

Each electrode attracts ions which are of the opposite charge. Therefore, positively charged ions (called cations) move towards the cathode, while negatively charged ions (termed anions) move toward the anode. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply. At the probes, electrons are absorbed or release by the ions, forming a collection of the desired element or compound.

One important use of electrolysis is to produce hydrogen. The reaction that occurs is:

2H20 (aq) → 2H2 (g) + O2 (g)

This has been suggested as a way of shifting society towards using hydrogen as an energy carrier for powering electric motors and internal combustion engines.

Electrolysis of water can be achieved in a simple hands-on project, where electricity from a battery is passed through a container of water (in practice a saltwater solution or other electrolyte will need to be used otherwise no result will be observed). Electrolysis of an aqueous solution of table salt (NaCl, or sodium choride) produces aqueous sodium hydroxide and chlorine, although usually only in minute amounts. NaCl (aq) can be reliably electrolysed to produce hydrogen. Hydrogen gas will be seen to bubble up at the cathode, and chlorine gas will bubble at the anode.

Big Idea. Water is comprised of two elements - hydrogen (H) and oxygen (O). Distilled water is pure and free of salts; thus it is a very poor conductor of electricity. By adding ordinary table salt (NaCl) to distilled water, it comes an electrolyte solution, able to conduct electricity.

Grade Level. High School.

Time Two 45 minute periods.

Content Standard. NSES Physical Science, properties and changes of properties in matter; atoms have measurable properties such as electrical charge.

Ocean Literacy Principle. (1e) Most of Earth's water (97%) is in the ocean. Seawater has unique properties: it is saline, its freezing point is slightly lower than fresh water, its density is slightly higher, its electrical conductivity is much higher, and it is slightly basic.

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